Nitrogen oxides (e.g., NOx) may be present in exhaust gases of a vehicle. NOx may form in engine cylinders when nitrogen (N2) and oxygen (O2) are exposed to elevated temperatures and pressures. The NOx may be processed via a selective reduction catalyst (SCR) in the vehicle's exhaust system into N2 and H2O. The SCR may co-operate with a reductant such as ammonia (NH3) to reduce the NOx when there is sufficient temperature within the SCR. However, if there is an insufficient amount of NH3 present at the SCR, a higher amount of NOx than is desirable may pass through the SCR. On the other hand, if excess NH3 is directed to or stored on the SCR, NH3 may slip past the SCR. Thus, it may be desirable to provide the SCR with an amount of NH3 that reduces a desirable portion of NOx from engine feedgas, yet is small enough to keep NH3 from slipping through the SCR.
One way to provide NH3 to a SCR is to model the engine feedgas emissions and model operation of the SCR. In a model based system, the NH3 may be released to the SCR (through urea injection control) based on the estimated operating state of the SCR. However, it may be difficult to accurately estimate chemical reactions, temperatures, and similar conditions for a SCR for a variety of reasons. On the other hand, rather than a model, a NOx sensor may be placed downstream of a SCR in an effort to determine if NOx is passing through a SCR. Nevertheless, NOx sensors have a cross sensitivity to NH3 so that the NOx sensor outputs a signal when NH3 is present in the absence or presence of NOx. Thus, the output of the NOx sensor may make it difficult to distinguish whether a SCR is slipping NOx or NH3.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for indicating a concentration of a gas, comprising: providing a NH3 concentration of a gas from an output of a first NOx sensor and an output of a second NOx sensor, the first and second NOx sensors having cross sensitivity between NOx and NH3, the first NOx sensor located upstream of the second NOx sensor in a direction of gas flow.
By considering an appropriately designed distribution that extracts the time dependent correlation in phase between the signals of the two NOx sensors it may be possible to distinguish between NOx and NH3. The sampled version of this distribution is designed to retain properties of both a short time Fourier Transform as well as the coherence function, thereby retaining both frequency correlation as well as phase information in time over short interval windows.
The present description may provide several advantages. For example, the approach may reduce engine NOx and NH3 emissions by providing feedback of NOx and NH3 exiting a SCR so that delivery of NH3 can be controlled responsive to use. Further, the approach may reduce system cost since both NOx and NH3 may be monitored via NOx sensors and without a NH3 sensor. The method may also reduce the amount of NH3 used within the SCR since NH3 injection can be reduced when NH3 is sensed.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.